1. Field of the Invention
The present invention relates to a load current output circuit for supplying a load current to an electronic device and an IC tester using the same load current output circuit and, particularly, to a dynamic load current output circuit for supplying or sinking a load current to or from an output terminal of an electronic device under test (DUT), which is capable of accurately measuring a signal rising time tr or a signal falling time tf when a rate of an output signal of the DUT becomes high and an IC tester using the same dynamic load current output circuit.
The load current output operation of the dynamic load current output circuit includes a pull operation to sink a current from the DUT.
2. Description of the Related Art
It has been known that an IC tester functions to preliminarily apply a predetermined voltage pulse signal to a predetermined terminal of an electronic device under test (DUT) and to judge, by a judge circuit thereof, whether a waveform of an output signal outputted from an output terminal (or an input/output terminal) of the DUT after a predetermined time from the application of the predetermined voltage pulse signal is in a high (H) level or a low (L) level according to a strobe signal generated with a predetermined timing. In this judge mode operation of the IC tester, when the IC tester measures the signal rising time tr or the falling time tf of the output signal of the DUT, the strobe signal having a predetermined period is applied to the judge circuit and a time from a time instance at which the strobe signal becomes xe2x80x9cLxe2x80x9d (or xe2x80x9cHxe2x80x9d) to a time instance at which it becomes xe2x80x9cHxe2x80x9d (or xe2x80x9cLxe2x80x9d) is measured. Further, an operation test or performance test, etc., of the DUT is performed by comparing the result of judgement with an expected value.
In the time measurement, in order to simulate a state where the DUT is connected to other device, a load current of a predetermined current value, for example, several mA to several tens mA, is preliminarily supplied to a specific output terminal of the DUT and the judgement of the level of the output signal of the DUT, xe2x80x9cHxe2x80x9d or xe2x80x9cLxe2x80x9d, is performed. In order to realize this, the IC tester is provided with a dynamic load current output circuit for supplying a load current to the output terminal of the DUT in the judge mode operation. The dynamic load current output circuit is usually connected to the output terminal of the DUT through a diode switch circuit provided therein.
Examples of the dynamic load current output circuit are disclosed in Japanese Patent Application Laid-open Nos. S62-116272, H2-128176 and H6-82527, etc
As shown in FIG. 5, which shows a relation of output current of a DUT and an output terminal voltage of the DUT, such dynamic load current output circuit pulls in a current IOH from the DUT when the output signal of the DUT is changed from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d and supplies a current IOL to the DUT when the output signal thereof is changed from xe2x80x9cHxe2x80x9d to xe2x80x9cLxe2x80x9d. Therefore, the IC tester can, in the IC test mode, simulate a value and a flowing direction of current in a state where some device is connected to the output terminal of the DUT.
The diode switch circuit is usually constructed with a diode bridge and supplies the load current to the DUT by ON/OFF controlling the diode correspondingly to the state of the output signal at the output signal terminal of the DUT.
FIG. 6 is a circuit diagram of a conventional dynamic load current output circuit 40 of an IC tester. As shown in FIG. 6, the dynamic load current output circuit 40, together with a driver 2 and a comparator 3, etc., is connected to a DUT 5 through a transmission line 1. Further, the dynamic load current output circuit 40 is connected between a positive power source +VCC and a negative power source xe2x88x92VEE and has a constant current source IOL for supplying a current IOL and a constant current source IOH for sinking a current IOH.
The comparator 3 is activated in the judge mode and, as mentioned previously, compares an input voltage with a reference voltage VTH according to a strobe signal generated with a predetermined timing and judges whether the input voltage level is xe2x80x9cHxe2x80x9d or xe2x80x9cLxe2x80x9d.
Incidentally, a reference numeral 4 denotes an input/output (I/O) switching circuit, I/O denotes a control bit signal for ON/OFF controlling the input/output switching circuit, 5a denotes an input/output (I/O) terminal of the DUT 5 and VS denotes a switching control voltage signal for the diode switch.
In the conventional IC tester constructed as mentioned above, there is a delay time xcfx84 of the transmission line 1 between the DUT 5 (an A point) and the dynamic load current output circuit 40 (a B point). Therefore, when the rising time tr of the output signal of the DUT 5 from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d becomes smaller than the delay time xcfx84 (tr less than xcfx84) due to increase of the frequency of the output signal of the DUT 5, the transition of the output signal of the DUT 5 from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d is completed before the dynamic load current output circuit 40 sinks the current IOH in response to the transition of the DUT 5. In such case, the dynamic load current output circuit 40 supplies the current IOL to the DUT 5 in the transition from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d. The current thus supplied to the DUT 5 functions to increase the rising rate of the output signal of the DUT 5. Therefore, the dynamic load current output circuit 40 not only performs its own function but also adversely affects the measurement.
This phenomenon is also true for the transition of the output signal of the DUT 5 from xe2x80x9cHxe2x80x9d to xe2x80x9cLxe2x80x9d. Therefore, when the frequency of the output signal of the DUT 5 is increased, it becomes impossible to precisely measure the signal rising time tr or the signal falling time tf of the output signal of the DUT5.
An object of the present invention is to provide a dynamic load current output circuit capable of precisely measuring the rising time tr or the falling time tf of an output signal of a DUT even when a frequency of the output signal is increased.
Another object of the present invention is to provide an IC tester using the same dynamic load current output circuit.
In order to achieve the above objects, the dynamic load current output circuit according to the present invention comprises a detector circuit for detecting a voltage level at an end of a transmission line, which is connected to the dynamic load current output circuit and receives an output signal from an electronic device under test through the transmission line or other transmission line. The dynamic load current output circuit pulls in a predetermined current from the transmission line or the other transmission line when the voltage level detected by the detection circuit is in low level and supplies a predetermined current equal to or different from the predetermined current to be pulled in to the transmission line or the other transmission line when the detected voltage level is in high level.
As mentioned above, the voltage level of the output signal of the electronic device under test at one end of the transmission line or the other transmission line, which is connected to the dynamic load current output circuit is detected by the detection circuit. When the currently detected voltage level of the output signal of the electronic device is xe2x80x9cLxe2x80x9d, a next voltage level transition at that point is to xe2x80x9cHxe2x80x9d. Therefore, when the output signal of the device under test is changed from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d, the dynamic load current output circuit preliminarily pulls in a predetermined current IOH from the transmission line prior to the transition time instance. When the currently detected voltage level is xe2x80x9cHxe2x80x9d, a next voltage level transition is to xe2x80x9cLxe2x80x9d. Therefore, when the output signal of the device under test is changed from xe2x80x9cHxe2x80x9d to xe2x80x9cLxe2x80x9d, the dynamic load current output circuit preliminarily supplies a predetermined current IOL to the transmission line prior to the transition time instance.
Therefore, in the dynamic load current output circuit and the IC tester having the same dynamic load current output circuit, which is connected to the electronic device under test through the transmission line, it is possible to supply a current to the electronic device or sinks a current therefrom when the output signal of the electronic device is changed from xe2x80x9cLxe2x80x9d to xe2x80x9cHxe2x80x9d or from xe2x80x9cHxe2x80x9d to xe2x80x9cLxe2x80x9d, without influence of the delay time of the transmission line.
As a result, it becomes possible to precisely measure the rising time tr or the falling time tf of the output signal of the electronic device even when the transition rate of the output signal of the electronic device becomes high.